Along with rapid development of national economy, cities have experienced growing power shortages. Newly built substations with voltages of 110 kV or higher are constantly emerging in suburban areas, economic development zones, and downtown areas. Old-fashioned outdoor substations with bracket structures can no longer meet the requirements because of considerations in urban planning, environmental protection, land resource conservation, reduction of routine maintenance and other factors. On the other hand, electrical device assembly has been widely adopted due to its advantages of small occupied floor space and suitability for indoor arrangement.
In the power industry, GIS (Gas Insulated Switchgear, hereinafter referred to as GIS) refers to an enclosed sulfur hexafluoride insulated electrical device assembly, internationally known as “gas insulated metal enclosed switchgear”. It combines all primary equipment except transformer in a substation, including circuit breaker, disconnecting switch, earthing switch, voltage transformer, current transformer, surge arrester, bus bar, cable terminal, incoming cable and outgoing cable bushings etc., into an integral assembly with an optimal design. GIS is also known as high voltage power distribution unit. Advantages of GIS lie in smaller occupied floor space, higher reliability, stronger safety, lower maintenance workload, and longer maintenance interval of major components of no less than 20 years.
Currently, production of GIS switchgear with voltages of 110 kV and higher has increased rapidly and has become a hot point. According to statistics provided by the National High Voltage Switch Association, production of 110 kV GIS in 2006 reached 3,664 bays, showing an increase of 993 bays from the previous year (2,671 bays) and an increase rate of 37.18%, and there were 15 GIS manufacturers, an increase of 4 from the previous year. The rapid development of 110 kV GIS is largely attributed to continuous modification and performance improvement on the products by manufacturers and a dramatic increase of demand from customers. Great progress has been made in the 110 kV GIS technology. Its structure has been improved from three phases in separate enclosures to three phases in one common enclosure. Its shell material has been evolved from steel plate to mostly aluminum alloys. Circuit break has been evolved from puffer type to self-extinguishing type. Disconnecting switch and earthing switch have been evolved to three-position combination type. Its operating mechanism has been evolved from hydraulic mechanism to light-duty spring operating mechanism; and some of secondary control systems have been evolved from conventional electro-magnetic type to intelligent electronic type. Some electric specifications have been enhanced from 126 kV/2000 A/31.5 kA to 145 kV/3150 A/40 kA. Technological advance has made 126 kV GIS smaller in size, lighter in weight, higher in reliability, and much lower in SF6 gas consumption. Therefore, it has gained a larger market share and has attracted more customers. GIS systems with voltages of 110 kV and higher have been widely used in large quantities and have been operated in some countries and regions for one or two decades. A lot of operation experiences have been gained. Manufacturers have been constantly improving GIS with voltages of 110 kV and higher based on production and operation experiences. Such improvement is mainly reflected in upgrade of specifications or renovation in structure to make the GIS even smaller in size and better in performance.
In the system, at the terminal of a transformer cable, a corona-free cap is usually used to improve electric field distribution around a conductive panel at the end of a transformer terminal and the transformer terminal, thus increasing their insulation level. A conventional corona-free cap is usually so designed that its connection interface (usually a hollow cavity) has the same size as the conductive panel on the epoxy insulator. Therefore, when a transformer terminal is electrically connected to transformer equipment, the first thing to do is to place the terminal for electrical connection on the conductive panel of the epoxy insulator so that it can contact directly with the conductive panel. Then, the connection interface of the corona-free cap is placed above the terminal and allowed to contact the terminal. Finally, the corona-free cap and the terminal are fixed on the conductive panel together with connecting bolts. However, a conventional corona-free cap must be placed above the terminal during installation. As a result, the corona-free cap must be removed before the terminal is connected with the corona-free cap during installation of the terminal. Therefore, installation of a conventional corona-free cap device is rather inconvenient. Moreover, if the terminal has already been connected with connecting wires before transformer equipment is shipped out of factory, an awkward situation would occur when the terminal is difficult to install since outer diameter of the joint with the terminal is larger than the bore size in the center of the corona-free cap.